A Long Term Evolution (LTE) system supports a time division duplexing (TDD) mode, that is, an uplink (UL) and a downlink (DL) use different time slots of a same frequency. An LTE TDD system may semi-statically configure uplink-downlink configurations according to service types, to meet different asymmetric uplink-downlink service requirements.
In the LTE TDD system, a used uplink-downlink configuration is semi-statically configured, and the configuration is changed at least every 640 milliseconds (ms), which may result in mismatching between a current uplink-downlink configuration and instantaneous uplink-downlink traffic. Therefore, resources cannot be utilized effectively, and this problem is especially serious for a cell with a relatively small number of user equipments. Therefore, in order to effectively improve a resource utilization rate, in a system of a new release, a TDD uplink-downlink configuration may be dynamically changed, for example, the uplink-downlink configuration is changed every 10 ms to 40 ms, and a base station (e.g., eNodeB (eNB)) notifies the TDD uplink-downlink configuration through a conventional physical downlink control channel (PDCCH) or an enhanced physical downlink control channel (ePDCCH). In the following, unless otherwise stated, a physical layer downlink control channel refers to the conventional physical downlink control channel or the enhanced physical downlink control channel (ePDCCH), and the physical layer downlink control channel may be abbreviated as (e)PDCCH. Because the (e)PDCCH is dynamic, the TDD uplink-downlink configuration can be dynamically changed. A user equipment that supports a function of dynamically changing the TDD uplink-downlink configuration is referred to as a further enhancements to LTE TDD for downlink-uplink interference management and traffic adaptation (eIMTA) user equipment, and is referred to as an eIMTA function-enabled user equipment in the specification for simplicity.
Because eIMTA user equipments and user equipments that do not enable an eIMTA function coexist in a communications network, where the user equipments that do not enable an eIMTA function include at least user equipments (UE) of releases prior to the 3rd Generation Partnership Project Release 12 (3GPP R12) and user equipments, which do not have an eIMTA function, of the 3GPP R12 and later. After sending a same preamble (also referred to as a prefix or a pilot) on a same random access channel (RACH) resource and receiving a random access message 2 (a random access response) on a same downlink resource, an eIMTA function-enabled user equipment and a non-eIMTA user equipment send random access messages 3 (which may be abbreviated as Msg3) according to a timing relationship specified in an existing protocol. The existing protocol specifies the following: It is assumed that the random access response is received in a subframe n, and then the Msg3 is sent in a first uplink subframe n+k1, where k1>=6, and subframes are labeled as 0 to 9. When a value of an uplink delay field in a random access response grant is 0, the subframe n+k1 is a first available uplink subframe (available UL subframe). When the value of the uplink delay field in the random access response grant is 1, the Msg3 is sent in a first available uplink subframe after the subframe n+k1. The eIMTA user equipment determines n+k1 according to a TDD uplink-downlink configuration notified on the (e)PDCCH, in other words, determines a subframe for sending the Msg3, according to a TDD uplink-downlink configuration notified on the (e)PDCCH, but the user equipment that does not enable an eIMTA function determines n+k1 according to a TDD uplink-downlink configuration notified in a system information block 1 (SIB1), in other words, determines a subframe for sending the Msg3, according to a TDD uplink-downlink configuration notified in a system information block 1 (system information block1, SIB1). Further, the TDD uplink-downlink configuration notified on the (e)PDCCH and the TDD uplink-downlink configuration notified in the SIB1 may be different. Because a base station does not know whether the user equipment that sends the preamble is an eIMTA function-enabled user equipment or a user equipment that does not enable an eIMTA function before correctly receiving the RACH Msg3, the base station does not know which configuration based on which the user equipment determines n+k1 or the uplink subframe for sending the Msg3. In this way, the user equipment that does not enable an eIMTA function, the eIMTA function-enabled user equipment, and the base station may have inconsistent understanding about the uplink subframe for sending the Msg3, so that it is possible that the base station fails to receive the corresponding Msg3.
In view of the above, in a random access process, how to enable a base station to accurately receive random access messages 3 sent by the eIMTA function-enabled user equipment and the user equipment that does not enable an eIMTA function becomes a problem that needs to be solved urgently at present.